The repeated consumption of habit-forming drugs such as alcohol, tranquilizers, stimulants, opiates, hallucinogens and nicotine, in many instances leads to some degree of addiction. Typically, such addiction is characterized by a desire or even the need to continue use of the drug and, in some cases, by a tendency to increase its dosage. Addiction results in a psychological and physiological dependence on the effects of such drugs and eventually has a detrimental effect on the addicted individual. The prevalence of drug addiction is well accepted as imposing significant costs on society. Two main categories of motivation in addiction are the desire to experience the hedonic (e.g., rewarding) effects of the drug of abuse and the desire to avoid the anhedonia or aversive consequences of drug withdrawal.
Withdrawal from the use of habit-forming drugs is difficult and presents a serious problem, in part due to the undesirable physical and/or psychologic symptoms that accompany the abstention. Both the rewarding aspect and the aversive withdrawal aspect of addiction have been studied, the mechanism of opiate addiction in particular having been reported upon. What becomes clear from the literature is that no single brain structure is entirely responsible for addiction and addictive behaviors. Repeated use of opiates induces long lasting changes in neural pathways and neural processing in many brain regions including but not limited to the nucleus accumbens, the ventral tegmental area, basolateral amygdala, locus coeruleus, and the bed nucleus of the stria terminalis. Supraspinal brain areas are also subject to modulation by ascending input from the spinal cord. There are profound effects of opiates on spinal neurotransmission. Regardless of brain or spinal cord region examined these long lasting changes include adaptation to neurotransmitter systems which include but are not limited to glutamatergic, dopaminergic, and adrenergic signaling. These adaptations may be involved in the reinforcing hedonic aspect of addiction as well as in the aversive reinforcing aspect of addiction. Opiates act on three classes of receptors (μ, κ, δ) with the μ-opioid receptor subtype being critical for the rewarding and aversive effects of opiates. One specific example of how opiates may mediate long lasting neuroadaptations of neural pathways and neural processing is described below.
One mechanism by which physical dependence to opiates manifests involves the noradrenergic cells of the locus ceruleus. Opiates act as agonists at inhibitory μ receptors on these cells, thereby decreasing presynaptic norepinephrine release by the cells. Over time, this results in an up-regulation of postsynaptic norepinephrine receptor expression. Concurrently, morphine down-regulates the synthesis of beta-endorphin, the normal endogenous agonist at the inhibitory μ receptors. When the opiate is withdrawn, the cell, no longer being inhibited, releases norepinephrine presynaptically. At the same time, postsynaptic supersensitivity, which results from the increase in norepinephrine receptors, leads to an amplification of the response, and an adrenergic storm ensues. This adrenergic storm manifests as a craving for more opiate, the ingestion of which re-starts and compounds the cycle.
The understanding of the central role of μ-opiate receptors in the mechanism of opiate addiction has led to several abstinence-oriented strategies to treat opiate addiction. One such abstinence-oriented strategy involves the regular, typically twice weekly, administration of naltrexone, a potent, orally-effective, long-lasting μ-receptor blocking agent. In another abstinence-oriented treatment, the opiate-dependent individual is maintained on buprenorphine. Because it is a partial μ-receptor agonist, buprenorphine has some slight reinforcing properties, and its acceptability by the opiate-dependent individual is high, as is compliance. At the same time, because it has high affinity for the μ-receptor, it blocks the effects of opiates and causes the opiate-dependent individual to stop seeking them.
Alcohol is another common drug of abuse, and a major public health problem worldwide. Few drugs exist that modulate the urge for alcohol intake and the molecular causes of alcoholism remain largely uncharacterized. Disulfram (ANTABUSE®) was introduced in 1951 for the treatment of alcoholism via inhibition of the enzyme aldehyde dehydrogenase (involved in the metabolism of alcohol to acetic acid); the drug causes headaches, dizziness and vomiting in the presence of alcohol, negatively reinforcing the urge for alcohol intake. Furthermore, administration of naltrexone, an opiate receptor antagonist, decreases alcohol self-administration in experimental animals and relapse in human alcoholics.
There is a continuing need for compounds that can alter consumption behavior by managing the withdrawal symptoms. Like opiates, neuroadaptations in many brain regions and neurotransmitter systems underlie the rewarding aspect and the aversive aspect of alcohol addiction. Similarly, supraspinal brain areas are also subject to modulation by ascending input from the spinal cord where alcohol exerts profound effects on spinal neurotransmission.
Protein kinase C (PKC) is a family of isozymes heavily involved in signal transduction cascades. As a variety of PKC isozymes are located throughout the neuroaxis (e.g., brain, spinal cord, and primary afferent neurons) and modulate actions downstream of neurotransmitters it is likely that PKC plays a role in the actions of drugs of abuse and in the generation of withdrawal symptoms. The PKC family of isozymes are key enzymes in signal transduction involved in a variety of cellular functions, including cell growth, regulation of gene expression, and ion channel activity.
The PKC family of isozymes includes at least eleven different protein kinases that can be divided into at least three subfamilies based on their homology and sensitivity to activators. Members of the classical or cPKC subfamily, α, βI, βII, and γPKC, contain four homologous domains (C1, C2, C3 and C4) inter-spaced with isozyme-unique (variable or V) regions, and require calciumand diacylglycerol for activation. Members of the classical PKC family are found in the superficial laminae of the dorsal horn in the spinal cord as well as in numerous brain regions. Members of the novel or nPKC subfamily, δ, ε, η, and θPKC, lack the C2 homologous domain and do not require calcium for activation. PKCε is found in primary afferent neuron terminals that innervate the spinal cord as well as in numerous brain regions. Finally, members of the atypical or αPKC subfamily, ζ and λ/iPKC, lack both the C2 and one half of the C1 homologous domains and are insensitive to diacylglycerol and calcium.
Studies on the subcellular distribution of PKC isozymes demonstrate that activation of PKC results in its redistribution in the cells (also termed translocation), such that activated PKC isozymes associate with the plasma membrane, cytoskeletal elements, nuclei, and other subcellular compartments (Saito, N. et al., Proc. Natl. Acad. Sci. USA, 86:3409-3413 (1989); Papadopoulos, V. and Hall, P. F. J. Cell Biol., 108:553-567 (1989); Mochly-Rosen, D., et al., Molec. Biol. Cell (formerly Cell Reg.), 1:693-706, (1990)). The unique cellular functions of different PKC isozymes are determined by their subcellular location. For example, activated βIPKC is found inside the nucleus, whereas activated βIIPKC is found at the perinucleus and cell periphery of cardiac myocytes (Disatnik, M. H., et al., Exp. Cell Res., 210:287-297 (1994)). The localization of different PKC isozymes to different areas of the cell in turn appears due to binding of the activated isozymes to specific anchoring molecules termed Receptors for Activated C-Kinase (RACKs). RACKs are thought to function by selectively anchoring activated PKC isozymes to their respective subcellular sites. RACKs bind only fully activated PKC and are not necessarily substrates of the enzyme. Nor is the binding to RACKs mediated via the catalytic domain of the kinase (Mochly-Rosen, D., et al., Proc. Natl. Acad. Sci. USA, 88:3997-4000 (1991)). Translocation of PKC reflects binding of the activated enzyme to RACKs anchored to the cell particulate fraction and the binding to RACKs is required for PKC to produce its cellular responses (Mochly-Rosen, D., et al., Science, 268:247-251 (1995)). Inhibition of PKC binding to RACKs in vivo inhibits PKC translocation and PKC-mediated function (Johnson, J. A., et al., J. Biol. Chem, 271:24962-24966 (1996a); Ron, D., et al., Proc. Natl. Acad. Sci. USA, 92:492-496 (1995); Smith, B. L. and Mochly-Rosen, D., Biochem. Biophys. Res. Commun., 188:1235-1240 (1992)).
In general, translocation of PKC is required for proper function of PKC isozymes. Peptides that mimic either the PKC-binding site on RACKs (Mochly-Rosen, D., et al., J. Biol. Chem., 226:1466-1468 (1991a); Mochly-Rosen, D., et al., supra, 1995) or the RACK-binding site on PKC (Ron, et al., supra, 1995; Johnson, J. A., et al., supra, 1996a) are isozyme-specific translocation inhibitors of PKC that selectively inhibit the function of the enzyme in vivo.
Agents capable of decreasing or overcoming such addiction and, if possible, alleviating or removing the symptoms related to the withdrawal of habit-forming and addictive drugs are desired by both persons suffering from addiction and by society in general. Inhibitors of PKC may be a class of such agents.